1. Field of the Invention
The invention relates to the field of optical communications, and more particularly to a variable optical attenuator for use in optical communication devices.
2. Description of the Prior Art
One of the key issues in optical communications is the regulation of optical signal power at both the input and output ports. At the input port, injection optical power has to be limited to a certain level to avoid unwanted nonlinear effects in optical fibers. At the output end, the optical power has to be controlled to within the limit of the dynamic range of the optical receiver. Further, the loss of optical energies during transmission in optical fibers and gains passing through fiber amplifiers, such as erbium-doped fiber amplifiers (EDFA), are generally wavelength dependent, and thus, if not modulated, the optical energies for different wavelengths would be different.
To achieve best performance for the optical network and, in the extreme case, to avoid damaging optical receivers, modulation of and sometimes flattening of optical power across spectral range is necessary. Modulators, particularly fixed optical modulators, such as fixed optical filters, are known in the art. However, such modulators, even if they are of the highest quality, generally are inadequate to address the problems of inappropriate power distribution across optical wavelengths. For example, in real-world optical networks, it is often necessary to quickly reconfigure a network, such as when it is necessary to recover from broken fibers or redirect network traffic to adjust for load. Fixed attenuators, or even manually adjustable attenuators, are not adequate for such quick network reconfiguration. Instead, variable optical attenuators must be used where the reduction of optical power for each wavelength can be dynamically adjusted as necessary, according to the detected power level at the wavelength, and sufficiently rapidly that users do not experience significant down time as networks are reconfigured.
In the prior art, modulation and flattening of optical powers has been performed by converting the optical signal to electrical signals, adjusting the electrical signals by mechanical or thermal mechanisms, and then regenerating the optical signals with suitable optical powers. This is recognized in the art as being a less than desirable solution, since it requires a more complex and more costly system than would be necessary if the modulation and flattening could be done optically. Thus, variable optical attenuators, such as liquid crystal and MEMs based optical attenuators that work in optical domains, is generally used in present-day fiber communication systems. However, while these attenuators are much faster than mechanically or thermally adjusted attenuators, they are not sufficient when dynamic network configuration must be performed faster than the millisecond range. In addition such attenuators are typically also bulky and polarization dependent.
Multiple quantum well structures (MQWS) and self-electrooptic effect devices (SEED) are known. See xe2x80x9cThe Quantum Well Self-Electrooptic Effect Device: Optoelectronic Bistability and Oscillation, and Self-Linearized Modulationxe2x80x9d, D. A. B. Miller et al., IEEE, J. Quantum Electronics QE21, 1462 (1985). MQW structures and SEED devices have been used in bistable logic devices, digital to analog converters, and modulators. See U.S. Pat. No. 4,751,378, Optical Device With Quantum Well Absorption, issued to Harvard S. Hinton et al. on Jun. 14, 1988; U.S. Pat. No. 4,851,840, Optical Analog To Digital Converter, issued to Alstair D. McAulay on Jul. 25, 1989; U.S. Pat. No. 5,323,019, All Optical Multiple Quantum Well Optical Modulator, issued to Mitra Dutla et al. on Jun. 21, 1994; and U.S. Pat. No. 6,115,375, Multistage Optical Packet Switching Apparatus Using Self Electro-Optic-Effect Devices, issued to Joon Kim Kwang et al. on Sep. 5, 2000.
The invention solves the above problems by providing a dynamically variable optical attenuator utilizing a multiple quantum well structure (MQWS), and a method of utilizing such an MQWS to attenuate light.
The optical attenuator according to the invention comprises an MQWS and a controller for controlling the attenuation in the MQWS. A light beam to be modulated is incident on the MQWS, is attenuated in the MQWS as determined by the controller, and then exits the MQWS. Preferably, the controller is a current controller. Preferably, the current controller comprises a current mirror. Preferably, the optical attenuator includes a photodetector, responsive to a portion of the exit optical beam, which photodetector provides a feedback signal to the controller. Preferably, the current controller includes a source of a reference signal and a comparator that compares the feedback signal from the photodetector to the reference signal to provide the output to the current mirror.
In the preferred embodiment, the MQWS is formed inside a photo intrinsic (PIN) diode to form a light modulator. The MQWS is preferably a stack of alternating layers of gallium arsenide and gallium aluminum arsenide. Radiation from an optical fiber is focused on the light modulator, preferably using an input lens. Generally, the optical absorption of the light modulator is proportional to the current through the device. Whatever radiation is not absorbed is transmitted and focused on an optical fiber, preferably using an output lens. A small, fixed portion of the transmitted radiation is directed to the photodetector, preferably via a beam splitter. Preferably, the output of the photodetector circuit is applied to a comparator which compares the signal from the photodetector circuit to a reference signal to provide a controller output to the current mirror to determine the current applied to the light modulator.
Preferably, the attenuator includes a plurality of MQWS devices, which the light passes through in series. Each MQWS device may be controlled by a separate control circuit, or a single control circuit may control all of the MQWS devices. The MQWS devices may be stacked; that is, all MQWS devices may be arranged serially along a line perpendicular to the layers of the MQWS.
Alternatively, the plurality of MQWS devices may be planar; that is, the MQWSs are all formed by the same GaAs and GaAlAs layers, with etched wells separating the devices. In this embodiment, optical prisms pass the radiation from one MQWS device to the next.
Preferably, a plurality of MQWS attenuators are formed into an array of MQWS channels between an optical demultiplexer and an optical multiplexer. The optical demultiplexer separates the input radiation into a spectrum of subbeams of different wavelengths, and each wavelength is passed through a different one of the MQWS attenuator channels. As known in the art, the attenuation wavelength of a MQWS device is determined by the thickness and materials, including dopings, of the MQWS layers. Each MQWS attenuator channel is designed to attenuate at a different narrow range of wavelengths, and preferably, is separately regulated. Each MQWS channel may be formed by a single MQWS attenuator. However, preferably, each channel is formed by a plurality of MQWS attenuators. The plurality of attenuators in each channel may be either stacked or planar.
In the most preferred form of the apparatus, the invention provides an optical attenuator for attenuating a light beam, the optical attenuator comprising: a multiple quantum well structure (MQWS), and an electro-optical feedback system responsive to the attenuated light beam and electrically connected to the MQWS for regulating the optical absorption of the MQWS.
In the most preferred form of the method, the invention provides a method of attenuating a light beam, the method comprising: providing a multiple quantum well structure (MQWS); directing a beam of light onto the MQWS; attenuating the light beam in the MQWS; exiting the light beam from the MQWS; directing a portion of the exit light beam onto an optical detection system, which in a simple case can be a photodetector, to provide a feedback signal; and utilizing the feedback signal to regulate the current in, and thus the attenuation power of, the MQWS.
In a preferred application of the attenuator, the invention provides an optical amplifier system that gives rise to an amplified optical beam, preferably with equal power across a whole spectrum. Preferably, the amplifier comprises: an optical amplifier; a multiple quantum well structure (MQWS); and an electrical circuit for supplying an electric current across the MQWS. Preferably, a plurality of MQWS attenuators are combined with an optical amplifier, such as an erbium-doped fiber amplifier (EDFA), to form an optical amplifier having a regulated output that is flatter than the typical output of an EDFA. Preferably, each of the attenuators is designed to attenuate at a different wavelength over the range of wavelengths amplified by the EDFA. In one embodiment, the plurality of MQWS attenuators are placed in series, and a variable grating, a photodetector, a sequencer and a multiplexer are used to sequentially tune each of the attenuators. In another embodiment, the attenuators are planar and are placed between an optical demultiplexer and an optical multiplexer, forming a plurality of attenuator channels which are separately regulated and then recombined at the optical multiplexer.
This invention provides a reliable dynamically variable optical attenuator that is polarization independent and that has a response time on the order of 10 nanoseconds (ns). In addition, the attenuator is completely solid state, can be fabricated via a mature manufacturing process, and is extremely compact in size. The extremely fast response time results in stable locking of the optical output signal. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.